1. General Background
The reproduction of color images on printing systems such as inkjet printers, toner based printing systems or offset presses relies on subtractive mixing of variable amounts of colorants. A typical set of colorants comprises cyan (C), magenta (M), yellow (Y) and black (K) colorants. In specific applications, such as the in the packaging industry, colorants may include other inks such as orange (O), green (G) or blue (B) inks, or inks having a different color.
The cost of colorants represents a considerable part of the total cost in print production. There is hence an economical incentive to reduce the cost of colorants by reducing colorant consumption.
Colorant consumption can be managed when an image is separated from color values into corresponding colorant values.
A typical prior art pre-press workflow uses Adobe Photoshop™. An original image is available from a photo scanner or a digital camera and represents an image represented in color vector space such sRGB or CIELab. In Photoshop, the image is separated and stored into a colorant vector space, for example a colorant space that corresponds with variable amounts of cyan, magenta, yellow and black inks on a press that meets the SWOP printing standard. For this purpose, Photoshop has access to a printer profile that characterizes such a printer. Usually a printer profile is described in an ICC (International Color Consortium) compliant format. Such a printer profile includes at least a “forward look up table” that tabulates the colors that result from printing a number of samples in the colorant vector space. The contents of the table are populated by printing and measuring a printer characterization target. Interpolation techniques are used to predict the color vector of a colorant vector that is not in the forward table.
For the purpose of converting a color vector into a colorant vector, an inverse table is used. This inverse table is usually populated by modeling the information in the forward table, and mathematically inverting this forward printer model. The inverse table can be included in a printer profile, or it can be calculated by a color management system from the forward table. The inverse table is used in combination with interpolation for the purpose of transforming a color vector into a colorant vector.
A user can impose certain additional parameters on the model inversion. A first restriction involves the selection of a rendering intent. A user can typically select between a calorimetric rendering and a perceptual rendering intent. In the first case, color vectors are converted into colorant vectors in a way that the printed color when measured, accurately corresponds with the original color vector. In the second case, an additional color distortion is permitted that takes into effect that the reproducible color gamut of a printer is usually different than the range of colors in an original image. The color distortion in perceptual rendering is such that it emphasizes preserving color differences, rather than that it aims to preserve the color itself.
Another set of additional parameters deals with setting the maximum value for the sum of the colorant values and the GCR (grey component replacement) settings.
Information on this and related subjects is found on the website of the International Color Committee (ICC).
2. The Problem when Documents have Different Origins
A problem originates when in a prepress organization a plurality of documents have to be processed in a workflow, wherein said documents have different origins.
Since the documents are already separated in colorants, but have different origins, there is absolutely no guarantee that the colorant set and the additional restrictions used for the model inversion are the same for all documents.
For a example, a first document may have been separated for a SWOP colorant set, using a GCR setting of 20% and a maximum for the sum of the colorant values equal to 340%, while a second document may have been separated using a GCR level of 70% and a maximum for the sum of the colorant values equal to 280%.
These different settings create an undesirable situation as they implicate that a printer has to set up his press for two different printing conditions to print both documents.
The situation is potentially even more complicated if two document objects are supplied that have been separated for different printing conditions but that are to be printed using the same print masters. In that case it is impossible to print both objects using the optimal press set-up. The optimal press set up in that case always involves making a compromise.
A first problem that hence needs to be resolved is to transform the colorant vectors in multiple document objects towards the same standardized printing process.
Since in general a considerable amount of colorant can be saved by increasing a level of GCR, a second problem that needs to be resolved is to transform the colorant vectors in multiple document objects in such a way that the total amount of colorant that is consumed during printing is reduced.
3. Prior Art Solutions
A prior art solution for resolving the above problems consists of making a color link by means of a color management system for transforming the original colorant vectors of an object into colorant vectors for the standardized printing process.
Such a link typically consists of a look up table which can be used in combination with an interpolation technique for transforming the colorant vectors of a raster image.
Making a link according to a prior art method involves the following steps:
First a forward look up table is identified having colorant vectors as entries and color vectors as data points.
Next the color vectors in the forward look up table are replaced by corresponding colorant vectors for the standardized printing process. For this purpose, a prior art technique uses an inverse look up table—having color vectors as entries and colorant vectors for the standardized printing process as data points—in combination with interpolation.
A first disadvantage of this prior art method is that it introduces interpolation error. This is particularly noticeable for the pure colorants, such as for example pure yellow, cyan or magenta. The conversion of these pure colorants from colorant to color vectors using the forward look up table may be fairly accurate because the pure colors correspond with sampling points of the forward interpolation table. The resulting color vectors, however, do generally not correspond with sampling points of the inverse look up table. This generally implies that the transformation from color vector to colorant vectors of the standardized printing process will introduce interpolation error for the colors that correspond with the pure colorants.
The net result is that, for example, a pure yellow colorant—after the forward and inverse transformations—will appear contaminated with cyan and magenta colorants. This may be unacceptable.